The invention relates to cellular phones used in mobile communications systems. More particularly, the invention relates to a circuit for controlling electrical characteristics of a signal transmitted by a cellular phone.
Usually, cellular phones are designed to operate within the environment of one of several mobile communications networks. One example of such an environment is a mobile communications network according to a standard known as GSM (Global System for Mobile communications) created by the European Telecommunications Standards Institute (ETSI). In GSM, there is an assigned frequency band around 900 MHz for Standard GSM, and an assigned frequency band around 1800 MHz for DCS1800 (Digital Communications System, DCS) which can be considered to be a further mobile communications environment. Other environments include systems known as Advanced Mobile Phone System (AMPS) operating in a frequency band around 800 MHz, and as Personal Communications System (PCS) operating in a frequency band around 1900 MHz.
As the number of users of cellular phones increases, some operators have added capacity to their networks by including more than only one frequency band in their networks. In the case of the GSM system, for example, the frequency bands around 900 MHz and 1800 MHz are now used to expand the capacities of certain networks.
Manufacturer of cellular phones, therefore, are developing a new generation of cellular phones which are operable at, for example, two frequency bands. A cellular phone operable at two frequency bands is referred to as a dual band cellular phone.
Independent of the kind of cellular phone, single band or dual band cellular phone, GSM requires that a transmitted radio frequency (RF) signal is accurately controlled, for example, in its power versus time profile. The transmitted power covers a dynamic range of up to 70 decibels (dB). Typically, the control is accomplished by means of a feedback control loop which samples the output power of the RF signal. Detectors used in such feedback control loops, however, have a limited dynamic range. This lack of dynamic range complicates the design of the feedback control loop.